1. Field of the Invention
The present invention relates to a semiconductor optical element which has a ridge-type structure or a high-mesa-type structure and a method for manufacturing the semiconductor optical element.
2. Background Art
Generally, a ridge-type optical waveguide or a high-mesa-type optical waveguide is used in a semiconductor optical element. In the ridge-type optical waveguide, light is confined in a horizontal direction by an upper clad layer which is etched to a mesa stripe shape. Since a core layer is not etched, the defect level of the semiconductor optical element can be reduced in the ridge-type optical waveguide. On the other hand, the high-mesa-type optical waveguide has a structure in which not only the upper clad layer but also the core layer and the lower clad layer are etched to a mesa stripe shape. The high-mesa-type structure has features of being capable of reducing an electrical capacitance when applied to a semiconductor optical element since the effect of the light confinement in the horizontal direction is large.
As just described, both of the semiconductor optical element having a high-mesa-type optical waveguide and the semiconductor optical element having a ridge-mesa-type optical waveguide have a mesa structure. The mesa structure is formed by wet etching. Advantages obtained by using the wet etching are mainly following two points. First, the etching can be stopped between two layers having a different composition from each other when an etchant and a semiconductor layer which is to be etched is properly selected. Second, the mesa structure which has a crystal face having a particular plane direction can be formed. Therefore a desired mesa structure can be formed by using the wet etching.
For example, there is a following description in “S. Adachi et al., Journal of The Electrochemical Society, vol. 129, no. 5, pp. 1053-1062(1982)”. That is, an n-InP clad layer, an undoped InGaAsP optical waveguide, a p-InP clad layer are formed one by one on an n-InP subsrate having a (001) face. Next, a SiO2 mask is formed on a wafer including the above layers. Next, the wafer is etched using HCl (hydrochloric acid): H3PO4 (phosphoric acid)=1:5-solution so that the p-InP clad layer is etched until the wet etching is stopped at a top surface of the InGaAsP optical waveguide layer. A side surface of the mesa structure formed by the etching is almost perpendicular to a (001) face when the wafer is cleaved in a (100) face. On the other hand, the side surface is inclined by 35 degrees to the (100) face when the wafer is cleaved in a (−110) face. In this way, a mesa structure is formed through the wet etching. Other methods for manufacturing the semiconductor optical element are disclosed in Patent Document 1-4.
Patent Document 1 is Japanese Patent Application Laid-Open No. 2004-327904, Patent Document 2 is Japanese Patent Application Laid-Open No. 2001-189523, Patent Document 3 is Japanese Patent Application Laid-Open No. 2004-071701, and Patent Document 4 is Japanese Patent Application Laid-Open No. 2002-198514.
When the wet etching is used as described above, the etching is stopped at a desired plane direction so that a mesa structure can be formed. However, when the mesa structure to be formed has a corner part, there is a problem that an abnormal etching proceeds in the corner part and a desired mesa structure can't be formed.
The corner part is, for example, a position at which two sides of the mesa structure intersect so as to form an angle of 90 degrees. As a concrete example of the abnormal etching, a ridge-type multimode interference (MMI) optical coupler will now be described. Upon manufacturing this ridge-type MMI optical coupler, a SiO2 mask is formed on a wafer which includes an n-InP subsrate as well as an n-InP clad layer, an undoped InGaAsP optical waveguide and a p-InP clad layer formed one by one on a (001) face of the n-InP subsrate. The n-InP clad layer of the wafer is etched using a solution of HCl: H3PO4=1:5-solution.
In the ridge-type MMI optical coupler, an input optical waveguide and an output optical waveguide are connected to a rectangular mesa structure. In the rectangular mesa structure, when a longer sideline is parallel to a (−110) face of the InP subsrate and a shorter sideline is parallel to a (110) face of the InP subsrate, a face (a perpendicular face) almost perpendicular to a (001) face is formed in the longer sideline and a face (a 35 degrees face) inclining by 35 degrees to a (001) face is formed in the shorter sideline. On “a corner part” at which the above described longer sideline and shorter sideline intersect with 90 degrees, the etching doesn't stop and but proceeds in the perpendicular surface and the 35 degrees surface. This etching proceeding in the corner part of the mesa structure is referred to as an anomal etching.
As a result of the abnormal etching in the corner part of the mesa structure, there arises a problem that the mesa structure is not formed as planned whereby a desired property is not given to the semiconductor optical element.